Remote control security systems are widely used in the automotive industry today. In addition to warning of an unauthorized entry into an automobile, such systems may allow the automobile owner to perform a variety of functions from a remote location, such as locking and unlocking the doors and trunk, raising and lowering the windows, starting the ignition, turning on and off the heat and air conditioning, and turning on and off headlights and interior lights.
Remote control security systems generally include a transmitting unit, which is portable, and a receiving unit, which is attached to the automobile. A binary verification code may be stored in both the transmitting unit and in the receiving unit. When a user presses a pushbutton on the transmitting unit, the verification code stored in the transmitting unit is transmitted as a series of pulses to the receiving unit. The receiving unit, upon receipt of transmission, compares the incoming verification code with a verification code stored in the receiving unit. If the verification code received from the transmitting unit matches the verification code stored in the receiving unit, then the receiving unit enables a function (for example unlocking the doors) to be performed. The function is dictated by a function code which is transmitted by the transmitting unit along with the verification code.
A drawback with the above described remote control security system is the ease of unauthorized entry into the automobile. With the proliferation of code copying devices, such as universal remote control units designed for combining into one remote control unit all the codes required to operate home entertainment systems, it has become possible to copy verification codes of automobile security systems. A universal remote control unit may receive a transmission from an original remote control unit of a television remote, for example, and may memorize that transmission so that the universal remote control unit can later retransmit the transmission to the television in the place of the original television remote.
In response to this problem, remote control security systems have been developed which use different verification codes on successive transmissions. These remote control security systems may include a counter in the transmitting unit and a corresponding counter in the receiving unit. The verification code may have two portions: the value output by the counter in the transmitting unit and a fixed system identification value. When the user presses the pushbutton on the transmitting unit, the value output by the counter in the transmitting unit is incremented and a verification code is transmitted incorporating both the new counter value and the fixed system identification value. The receiving unit, after receiving the incoming transmission, compares a fixed system identification value stored in the receiving unit with the fixed system identification value received. If the fixed system identification values match, then the counter in the receiving unit is incremented and the output of the incremented counter in the receiving unit is compared with the counter value received. The counter values of the counter in the transmitting unit and the counter in the receiving unit should therefore match each other on successive transmissions if the fixed system identification code transmitted and the fixed system identification code stored in the receiving unit match. Due to the operation of the counters, a thief's copying of a verification code of a given transmission will not enable the thief to gain entry to the automobile by retransmitting the verification code because the receiving unit expects a different verification code for the next transmission. The counter may be a pseudo-random number generator to make deciphering the next counter value more difficult if the present counter value is known.
Although conventional remote control security systems which use incrementing counter values in conjunction with verification codes are generally more secure than systems which always transmit the same verification code, car thieves may still be able to gain unauthorized entry into automobiles due to an inherent weakness of these conventional systems. When power is disconnected and then resupplied to the transmitting unit of one of these security systems (for example, by removing and then reinserting the battery), the counter of the transmitting unit typically restarts at an initial count value. Accordingly, the first verification code after the battery is removed and re-inserted is always the same, the second verification code after the battery is removed and re-installed is always the same, and so on. This predictability has lead to a risk of unauthorized access.
Consider the following valet parking scenario. When parking an automobile equipped with such a remote control security system, the valet parking attendant is given custody of the transmitting unit of the security system along with the mechanical keys to the automobile. If the valet is in possession of a code copying device, the valet may obtain access to the automobile at some time in the future when the automobile is no longer in the custody of the parking attendant and when thievery is not directly traceable to the attendant. First, the valet removes and re-inserts the battery in the transmitting unit, thus resetting the counter to its initial value. Second, the valet repeatedly presses the button on the transmitting unit and uses a code copier to copy the next certain number (for example fifty) of verification codes. Third, the valet then once again removes and re-inserts the battery to reset the counter in the transmitting unit back to its initial value. Fourth, the valet resynchronizes the receiving unit to the reset transmitting unit.
Now in possession of the next certain number (for example fifty) of successive verification codes of the security system, the valet may be able to locate the automobile on the street after the automobile has left valet custody and to transmit the successive verification codes stored in the code copier. If the car owner has only used the transmitting unit a small number of times (for example, less than fifty) since the verification codes were copied, then the verification code expected by the receiving unit will be one of the (fifty) verification codes in the possession of the thief. The thief may therefore be able to gain access to the automobile by transmitting successive verification codes until the correct verification code opens the car.